This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 arising from an application entitled, WAVELENGTH STABILIZER IN WDM OPTICAL TRANSMISSION SYSTEM, filed earlier in the Korean Industrial Property Office on Oct. 22, 1998, and there duly assigned Serial No. 1998-44296.
1. Field of the Invention
The present invention relates to an optical transmission system using a wavelength division multiplex (WDM) scheme, and in particular, to a wavelength stabilizer for stabilizing the wavelength of an optical signal generated by a laser diode (LD) in each channel transmitter.
2. Description of the Related Art
WDM is one form of optical signal transmitting schemes, in which a plurality of optical signals at different wavelengths is simultaneously propagated through a strand of optical fiber. Multiplexing optical signals at different wavelengths on a strand of optical fiber is called wavelength division multiplexing, which is commonly identified by the acronym WDM, and the reverse operation is called xe2x80x9cwavelength division demultiplexing.xe2x80x9d
In a WDM optical transmission system, a laser transmitter having an LD with a narrow spectrum as a light source is used as a channel transmitter, and channels are densely arranged in approximately 100 or 200 GHz intervals. The wavelength of an optical signal oscillated by such an LD should be within the range predetermined for a corresponding channel.
In the prior art, the wavelength of an optical signal within a predetermined range is stabilized based two different methods.
The first method utilizes the principle that the oscillation wavelength of an LD depends on an operating temperature. To stabilize the wavelength of an optical signal oscillated by the LD, the resistance of a thermistor sensor provided in the LD is compared with a reference value corresponding to a predetermined temperature, and an operating temperature is controlled according to the comparison result. A thermoelectric cooler (TEC) is typically used for the temperature control. This method is advantageous in that the control mechanism is simple, there is no need for additional parts to monitor wavelength deviation, and a high level of stability can be achieved. However, the LD deteriorates with time and thus the threshold current value increases with passage of time. Therefore, a driving circuit of the laser transmitter is made to maintain the optical output power constant by automatically controlling a bias current value. Despite the maintenance of a constant operating temperature, the wavelength of an oscillated optical signal gradually deviates when the LD is used for a long time, and it finally deviates from the predetermined range even with an accurate control of wavelength based on the predetermined temperature.
The second method requires the use of a separately procured wavelength stabilizing device for detecting the wavelength of an optical signal. The wavelength stabilizing device detects how much the wavelength of an optical signal deviates from a predetermined range and the temperature of an LD is controlled in real time. The wavelength stabilizing device may be used for each wavelength division multiplexed channel by extracting signals from a wavelength division multiplexed (WDM) optical signal. This method has the advantage of wavelength stabilization based on the accurate wavelength deviation information, but suffers from the constraint of obtaining the stability of a real time controlling circuit and also suffers from high cost due to the use of an expensive wavelength-stabilizing device for each channel.
As described above, the conventional wavelength stabilizing methods known in the prior art are not effective in stabilizing the wavelength of an optical signal in the long term, or have problems with the stability of a real time controlling circuit and cause high cost due to the requirement of a separately procured device.
Therefore, an objective of the present invention is to provide a wavelength stabilizer, which can simply and cheaply compensate for the deviation in the wavelength of an optical signal and ensure wavelength stability in the long term.
To achieve the above objective, this invention provides a single wavelength stabilizer that is operable to affect the LD in each of the plurality of channel transmitters in a WDM, in which a temperature controlling circuit in each channel transmitter senses an LD operating temperature and maintains the LD at a reference temperature, a wavelength monitor is connected to the wavelength division multiplexer by an optical transmission line for monitoring the wavelength deviation of an optical signal for each channel in the multiplexed signal, further outputs the monitored wavelength deviation information, and a controller determines the wavelength deviation of each channel from the wavelength deviation information and updates the reference temperature of the temperature controlling circuit of each channel transmitter in order to control the operating temperature of the LD.